Hypersomnia is an uncommon disorder. In general, 5% or fewer of adults complain of excessive sleepiness during daytime. That does not mean all those who complain of excessive sleepiness suffer from hypersomnia. There are many other possible causes of daytime sleepiness. Among all the people who visit sleep clinics with complaints of excessive daytime sleepiness, only about 5-10% actually account for Primary Hypersomnia
Hypersomnia usually appears when the patient is 15 - 30 years old. It does not begin suddenly, but becomes apparent slowly, sometimes with years. Unfortunately, as with most sleep disorders, hypersomnia too is under- reported, because many people believe that feeling sleepy and taking naps are normal behavior.
Kleine-Levin Syndrome begins during adolescence, may continue for years, and may disappear by itself during adulthood. Kleine-Levin Syndrome affects males approximately 3 times more often than females. The onset of this condition is common during adolescence and rare in people older than 30 years.